The goal of accessing data from a drill string has been expressed for more than half a century. As exploration and drilling technology has improved, this goal has become more important in the industry for successful oil, gas, and geothermal well exploration and production. For example, to take advantage of the several advances in the design of various tools and techniques for oil and gas exploration, it would be beneficial to have real time data such as temperature, pressure, inclination, salinity, etc. Several attempts have been made to devise a successful system for accessing such drill string data. However, due to the complexity, expense, and unreliability of such systems, many attempts to create such a system have failed to achieve significant commercial acceptance.
In U.S. Pat. No. 6,670,880 issued to Hall et al., the inventors disclosed a “downhole transmission system” that overcomes many of the problems and limitations of the prior art. In the Hall patent, data is transmitted along the drill string in real time. This is accomplished by various transmission hardware components integrated directly into the drill string. The Hall patent allows various tools and sensors, located downhole, to communicate with surface equipment in real time.
One problem that has not been adequately addressed involves the time synchronization of downhole components. Time synchronization is important because with some tools and sensors, it is very important to know precisely when particular data is gathered or measured. For example, precisely synchronized clocks are very important when taking seismic measurements downhole. Downhole seismic surveys (also called borehole velocity surveys) are conducted by measuring the time for seismic waves, generated by a seismic impulse, to travel to sensors located at different depths or locations underground. In order to extract usable information (i.e. create useful seismic images or the like) from data gathered by these sensors, the data must be accurately time-stamped.
Synchronizing the time of downhole devices may be further complicated by the very nature of a downhole environment. For example, oscillators that form the basis for many clocks used in electronic equipment often vary in output frequency. Thus, clocks may gradually drift away from a desired reference time. This drift is worsened in downhole environments because extreme temperatures, whether hot or cold, often amplify fluctuations in oscillator frequency. Thus, clocks that provide adequate precision above the surface may be unsuitable in downhole drilling applications.
Accordingly, what are needed are accurate clocks that maintain their accuracy in downhole environments. What are further needed are apparatus and methods to more effectively compensate for drift in clocks utilizing conventional oscillators. Further needed are clocks that are sufficiently accurate for precision-demanding applications such as downhole seismic devices. Finally, what are needed are apparatus and methods for synchronizing downhole clocks with a reference time source.